Smooth muscle regulates airway diameter in gas exchange

Steps:

• Identify key structures in transverse section of bronchi/bronchioles: cartilage for support, smooth muscle for control.
• Recall smooth muscle location: circular layers in bronchial walls, labeled likely as Q.
• Match function: smooth muscle contracts/relaxes to adjust airway diameter, regulating airflow to alveoli.
• Eliminate mismatches: incorrect labels or functions point to other structures like cartilage or skeletal muscles.

Why D is correct:

• Smooth muscle controls bronchoconstriction/dilation, regulating airflow resistance per physiological definition of autonomic regulation in respiratory tract.

Why the others are wrong:

• A: Lacks specific label (e.g., P or Q), making it incomplete.
• B: "The gas pumps air" misattributes pumping to gas (likely error for cartilage), not smooth muscle function.
• C: Pumping air describes diaphragm/skeletal muscle action, not local smooth muscle regulation.

Final answer: D